Network Working Group A. Bittau
Internet-Draft Google
Intended status: Experimental D. Giffin
Expires: April 24, 2018 Stanford University
M. Handley
University College London
D. Mazieres
Stanford University
Q. Slack
Sourcegraph
E. Smith
Kestrel Institute
October 21, 2017
Cryptographic protection of TCP Streams (tcpcrypt)
draft-ietf-tcpinc-tcpcrypt-08
Abstract
This document specifies tcpcrypt, a TCP encryption protocol designed
for use in conjunction with the TCP Encryption Negotiation Option
(TCP-ENO). Tcpcrypt coexists with middleboxes by tolerating
resegmentation, NATs, and other manipulations of the TCP header. The
protocol is self-contained and specifically tailored to TCP
implementations, which often reside in kernels or other environments
in which large external software dependencies can be undesirable.
Because the size of TCP options is limited, the protocol requires one
additional one-way message latency to perform key exchange before
application data may be transmitted. However, this cost can be
avoided between two hosts that have recently established a previous
tcpcrypt connection.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
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This Internet-Draft will expire on April 24, 2018.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Encryption Protocol . . . . . . . . . . . . . . . . . . . . . 3
3.1. Cryptographic Algorithms . . . . . . . . . . . . . . . . 3
3.2. Protocol Negotiation . . . . . . . . . . . . . . . . . . 5
3.3. Key Exchange . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Session ID . . . . . . . . . . . . . . . . . . . . . . . 8
3.5. Session Resumption . . . . . . . . . . . . . . . . . . . 9
3.6. Data Encryption and Authentication . . . . . . . . . . . 12
3.7. TCP Header Protection . . . . . . . . . . . . . . . . . . 13
3.8. Re-Keying . . . . . . . . . . . . . . . . . . . . . . . . 13
3.9. Keep-Alive . . . . . . . . . . . . . . . . . . . . . . . 14
4. Encodings . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. Key-Exchange Messages . . . . . . . . . . . . . . . . . . 15
4.2. Encryption Frames . . . . . . . . . . . . . . . . . . . . 17
4.2.1. Plaintext . . . . . . . . . . . . . . . . . . . . . . 17
4.2.2. Associated Data . . . . . . . . . . . . . . . . . . . 18
4.2.3. Frame Nonce . . . . . . . . . . . . . . . . . . . . . 19
4.3. Constant Values . . . . . . . . . . . . . . . . . . . . . 19
5. Key-Agreement Schemes . . . . . . . . . . . . . . . . . . . . 19
6. AEAD Algorithms . . . . . . . . . . . . . . . . . . . . . . . 20
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
8. Security Considerations . . . . . . . . . . . . . . . . . . . 21
8.1. Asymmetric Roles . . . . . . . . . . . . . . . . . . . . 23
8.2. Verified Liveness . . . . . . . . . . . . . . . . . . . . 23
8.3. Mandatory Key-Agreement Schemes . . . . . . . . . . . . . 24
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24
10. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 25
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
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11.1. Normative References . . . . . . . . . . . . . . . . . . 25
11.2. Informative References . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27
1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Introduction
This document describes tcpcrypt, an extension to TCP for
cryptographic protection of session data. Tcpcrypt was designed to
meet the following goals:
o Meet the requirements of the TCP Encryption Negotiation Option
(TCP-ENO) [I-D.ietf-tcpinc-tcpeno] for protecting connection data.
o Be amenable to small, self-contained implementations inside TCP
stacks.
o Minimize additional latency at connection startup.
o As much as possible, prevent connection failure in the presence of
NATs and other middleboxes that might normalize traffic or
otherwise manipulate TCP segments.
o Operate independently of IP addresses, making it possible to
authenticate resumed sessions efficiently even when either end
changes IP address.
A companion document [I-D.ietf-tcpinc-api] describes recommended
interfaces for configuring certain parameters of this protocol.
3. Encryption Protocol
This section describes the tcpcrypt protocol at an abstract level.
The concrete format of all messages is specified in Section 4.
3.1. Cryptographic Algorithms
Setting up a tcpcrypt connection employs three types of cryptographic
algorithms:
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o A _key agreement scheme_ is used with a short-lived public key to
agree upon a shared secret.
o An _extract function_ is used to generate a pseudo-random key
(PRK) from some initial keying material, typically the output of
the key agreement scheme. The notation Extract(S, IKM) denotes
the output of the extract function with salt S and initial keying
material IKM.
o A _collision-resistant pseudo-random function (CPRF)_ is used to
generate multiple cryptographic keys from a pseudo-random key,
typically the output of the extract function. The CPRF produces
an arbitrary amount of Output Keying Material (OKM), and we use
the notation CPRF(K, CONST, L) to designate the first L bytes of
the OKM produced by the CPRF when parameterized by key K and the
constant CONST.
The Extract and CPRF functions used by the tcpcrypt variants defined
in this document are the Extract and Expand functions of HKDF
[RFC5869], which is built on HMAC [RFC2104]. These are defined as
follows in terms of the function "HMAC-Hash(key, value)" for a
negotiated "Hash" function such as SHA-256; the symbol | denotes
concatenation, and the counter concatenated to the right of CONST
occupies a single octet.
HKDF-Extract(salt, IKM) -> PRK
PRK = HMAC-Hash(salt, IKM)
HKDF-Expand(PRK, CONST, L) -> OKM
T(0) = empty string (zero length)
T(1) = HMAC-Hash(PRK, T(0) | CONST | 0x01)
T(2) = HMAC-Hash(PRK, T(1) | CONST | 0x02)
T(3) = HMAC-Hash(PRK, T(2) | CONST | 0x03)
...
OKM = first L octets of T(1) | T(2) | T(3) | ...
where L < 255*OutputLength(Hash)
Figure 1: HKDF functions used for key derivation
Lastly, once tcpcrypt has been successfully set up and encryption
keys have been derived, an algorithm for Authenticated Encryption
with Associated Data (AEAD) is used to protect the confidentiality
and integrity of all transmitted application data. AEAD algorithms
use a single key to encrypt their input data and also to generate a
cryptographic tag to accompany the resulting ciphertext; when
decryption is performed, the tag allows authentication of the
encrypted data and of optional, associated plaintext data.
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3.2. Protocol Negotiation
Tcpcrypt depends on TCP-ENO [I-D.ietf-tcpinc-tcpeno] to negotiate
whether encryption will be enabled for a connection, and also which
key-agreement scheme to use. TCP-ENO negotiates the use of a
particular TCP encryption protocol or _TEP_ by including protocol
identifiers in ENO suboptions. This document associates four TEP
identifiers with the tcpcrypt protocol, as listed in Table 2. Each
identifier indicates the use of a particular key-agreement scheme,
with an associated CPRF and length parameters. Future standards may
associate additional TEP identifiers with tcpcrypt, following the
assignment policy specified by TCP-ENO.
An active opener that wishes to negotiate the use of tcpcrypt
includes an ENO option in its SYN segment. That option includes
suboptions with tcpcrypt TEP identifiers indicating the key-agreement
schemes it is willing to enable. The active opener MAY additionally
include suboptions indicating support for encryption protocols other
than tcpcrypt, as well as global suboptions as specified by TCP-ENO.
If a passive opener receives an ENO option including tcpcrypt TEPs it
supports, it MAY then attach an ENO option to its SYN-ACK segment,
including _solely_ the TEP it wishes to enable.
To establish distinct roles for the two hosts in each connection,
tcpcrypt depends on the role-negotiation mechanism of TCP-ENO. As
one result of the negotiation process, TCP-ENO assigns hosts unique
roles abstractly called "A" at one end of the connection and "B" at
the other. Generally, an active opener plays the "A" role and a
passive opener plays the "B" role; but in the case of simultaneous
open, an additional mechanism breaks the symmetry and assigns a
distinct role to each host. TCP-ENO uses the terms "host A" and
"host B" to identify each end of a connection uniquely, and this
document employs those terms in the same way.
An ENO suboption includes a flag "v" which indicates the presence of
associated, variable-length data. In order to propose fresh key
agreement with a particular tcpcrypt TEP, a host sends a one-byte
suboption containing the TEP identifier and "v = 0". In order to
propose session resumption (described further below) with a
particular TEP, a host sends a variable-length suboption containing
the TEP identifier, the flag "v = 1", and an identifier derived from
a session secret previously negotiated with the same host and the
same TEP.
Once two hosts have exchanged SYN segments, TCP-ENO defines the
_negotiated TEP_ to be the last valid TEP identifier in the SYN
segment of host B (that is, the passive opener in the absence of
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simultaneous open) that also occurs in that of host A. If there is
no such TEP, hosts MUST disable TCP-ENO and tcpcrypt.
If the negotiated TEP was sent by host B with "v = 0", it means that
fresh key agreement will be performed as described below in
Section 3.3. If it had "v = 1", the key-exchange messages will be
omitted in favor of determining keys via session-resumption as
described in Section 3.5, and protected application data may
immediately be sent as detailed in Section 3.6.
Note that the negotiated TEP is determined without reference to the
"v" bits in ENO suboptions, so if host A offers resumption with a
particular TEP and host B replies with a non-resumption suboption
with the same TEP, that may become the negotiated TEP and fresh key
agreement will be performed. That is, sending a resumption suboption
also implies willingness to perform fresh key agreement with the
indicated TEP.
As required by TCP-ENO, once a host has both sent and received an ACK
segment containing a valid ENO option, encryption MUST be enabled and
plaintext application data MUST NOT ever be exchanged on the
connection. If the negotiated TEP is among those listed in Table 2,
a host MUST follow the protocol described in this document.
3.3. Key Exchange
Following successful negotiation of a tcpcrypt TEP, all further
signaling is performed in the Data portion of TCP segments. Except
when resumption was negotiated (described below in Section 3.5), the
two hosts perform key exchange through two messages, "Init1" and
"Init2", at the start of the data streams of host A and host B,
respectively. These messages may span multiple TCP segments and need
not end at a segment boundary. However, the segment containing the
last byte of an "Init1" or "Init2" message MUST have TCP's push flag
(PSH) set.
The key exchange protocol, in abstract, proceeds as follows:
A -> B: Init1 = { INIT1_MAGIC, sym-cipher-list, N_A, PK_A }
B -> A: Init2 = { INIT2_MAGIC, sym-cipher, N_B, PK_B }
The concrete format of these messages is specified in Section 4.1.
The parameters are defined as follows:
o "INIT1_MAGIC", "INIT2_MAGIC": constants defined in Table 1.
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o "sym-cipher-list": a list of symmetric ciphers (AEAD algorithms)
acceptable to host A. These are specified in Table 3.
o "sym-cipher": the symmetric cipher selected by host B from the
"sym-cipher-list" sent by host A.
o "N_A", "N_B": nonces chosen at random by hosts A and B,
respectively.
o "PK_A", "PK_B": ephemeral public keys for hosts A and B,
respectively. These, as well as their corresponding private keys,
are short-lived values that SHOULD be refreshed periodically. The
private keys SHOULD NOT ever be written to persistent storage.
The ephemeral secret ("ES") is the result of the key-agreement
algorithm (see Section 5) indicated by the negotiated TEP. The
inputs to the algorithm are the local host's ephemeral private key
and the remote host's ephemeral public key. For example, host A
would compute "ES" using its own private key (not transmitted) and
host B's public key, "PK_B".
The two sides then compute a pseudo-random key ("PRK"), from which
all session keys are derived, as follows:
PRK = Extract(N_A, eno-transcript | Init1 | Init2 | ES)
Above, "|" denotes concatenation; "eno-transcript" is the protocol-
negotiation transcript defined in Section 4.8 of
[I-D.ietf-tcpinc-tcpeno]; and "Init1" and "Init2" are the transmitted
encodings of the messages described in Section 4.1.
A series of "session secrets" are then computed from "PRK" as
follows:
ss[0] = PRK
ss[i] = CPRF(ss[i-1], CONST_NEXTK, K_LEN)
The value "ss[0]" is used to generate all key material for the
current connection. The values "ss[i]" for "i > 0" can be used to
avoid public key cryptography when establishing subsequent
connections between the same two hosts, as described in Section 3.5.
The "CONST_*" values are constants defined in Table 1. The length
"K_LEN" depends on the tcpcrypt TEP in use, and is specified in
Section 5.
Given a session secret "ss[i]", the two sides compute a series of
master keys as follows:
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mk[0] = CPRF(ss[i], CONST_REKEY, K_LEN)
mk[j] = CPRF(mk[j-1], CONST_REKEY, K_LEN)
The process of advancing through the series of master keys is
described in Section 3.8.
Finally, each master key "mk[j]" is used to generate keys for
authenticated encryption:
k_ab[j] = CPRF(mk[j], CONST_KEY_A, ae_keylen)
k_ba[j] = CPRF(mk[j], CONST_KEY_B, ae_keylen)
In the first session derived from fresh key-agreement, keys "k_ab[j]"
are used by host A to encrypt and host B to decrypt, while keys
"k_ba[j]" are used by host B to encrypt and host A to decrypt. In a
resumed session, as described more thoroughly below in Section 3.5,
each host uses the keys in the same way as it did in the original
session, regardless of its role in the current session: for example,
if a host played role "A" in the first session, it will use keys
"k_ab[j]" to encrypt in each derived session.
The value "ae_keylen" depends on the authenticated-encryption
algorithm selected, and is given under "Key Length" in Table 3.
After host B sends "Init2" or host A receives it, that host may
immediately begin transmitting protected application data as
described in Section 3.6.
If host A receives "Init2" with a "sym-cipher" value that was not
present in the "sym-cipher-list" it previously transmitted in
"Init1", it MUST abort the connection and raise an error condition
distinct from the end-of-file condition.
Throughout this document, to "abort the connection" means to issue
the "Abort" command as described in [RFC0793], Section 3.8. That is,
the TCP connection is destroyed, RESET is transmitted, and the local
user is alerted to the abort event.
3.4. Session ID
TCP-ENO requires each TEP to define a _session ID_ value that
uniquely identifies each encrypted connection.
As required, a tcpcrypt session ID begins with the byte transmitted
by host B that contains the negotiated TEP identifier along with the
"v" bit. The remainder of the ID is derived from the session secret,
as follows:
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session_id[i] = TEP-byte | CPRF(ss[i], CONST_SESSID, K_LEN)
Again, the length "K_LEN" depends on the TEP, and is specified in
Section 5.
3.5. Session Resumption
If two hosts have previously negotiated a session with a particular
session secret, they can establish a new connection without public-
key operations using the next session secret in the sequence derived
from the original PRK.
A host signals willingness to resume with a particular session secret
by sending a SYN segment with a resumption suboption: that is, an ENO
suboption whose value is the negotiated TEP identifier of the session
concatenated with half of the "resumption identifier" for the
session.
The resumption identifier is calculated from a session secret "ss[i]"
as follows:
resume[i] = CPRF(ss[i], CONST_RESUME, 18)
To name a session for resumption, a host sends either the first or
second half of the resumption identifier, according to the role it
played in the original session with secret "ss[0]".
A host that originally played role A and wishes to resume from a
cached session sends a suboption with the first half of the
resumption identifier:
byte 0 1 9 (10 bytes total)
+--------+--------+---...---+--------+
| TEP- | resume[i]{0..8} |
| byte | |
+--------+--------+---...---+--------+
Figure 2: Resumption suboption sent when original role was A. The
TEP-byte contains a tcpcrypt TEP identifier and v = 1.
Similarly, a host that originally played role B sends a suboption
with the second half of the resumption identifier:
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byte 0 1 9 (10 bytes total)
+--------+--------+---...---+--------+
| TEP- | resume[i]{9..17} |
| byte | |
+--------+--------+---...---+--------+
Figure 3: Resumption suboption sent when original role was B. The
TEP-byte contains a tcpcrypt TEP identifier and v = 1.
If a passive opener receives a resumption suboption containing an
identifier-half it recognizes as being derived from a session secret
that it has cached, it SHOULD (with exceptions specified below) agree
to resume from the cached session by sending its own resumption
suboption, which will contain the other half of the identifier.
If the passive opener does not agree to resumption with a particular
TEP, it may either request fresh key exchange by responding with a
non-resumption suboption using the same TEP, or else respond to any
other received suboption.
If an active opener receives a resumption suboption for a particular
TEP and the received identifier-half does not match the "resume[i]"
value whose other half it previously sent in a resumption suboption
for the same TEP, it MUST ignore that suboption. In the typical case
that this was the only ENO suboption received, this means the host
MUST disable TCP-ENO and tcpcrypt: that is, it MUST NOT send any more
ENO options and MUST NOT encrypt the connection.
When a host concludes that TCP-ENO negotiation has succeeded for some
TEP that was received in a resumption suboption, it MUST then enable
encryption with that TEP, using the cached session secret, as
described in Section 3.6.
The session ID (Section 3.4) is constructed in the same way for
resumed sessions as it is for fresh ones. In this case the first
byte will always have "v = 1". The remainder of the ID is derived
from the cached session secret.
In the case of simultaneous open where TCP-ENO is able to establish
asymmetric roles, two hosts that simultaneously send SYN segments
with compatible resumption suboptions may resume the associated
session.
In a particular SYN segment, a host SHOULD NOT send more than one
resumption suboption, and MUST NOT send more than one resumption
suboption with the same TEP identifier. But in addition to any
resumption suboptions, an active opener MAY include non-resumption
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suboptions describing other TEPs it supports (in addition to the TEP
in the resumption suboption).
After using "ss[i]" to compute "mk[0]", implementations SHOULD
compute and cache "ss[i+1]" for possible use by a later session, then
erase "ss[i]" from memory. Hosts SHOULD retain "ss[i+1]" until it is
used or the memory needs to be reclaimed. Hosts SHOULD NOT write a
cached "ss[i+1]" value to non-volatile storage.
When proposing resumption, the active opener MUST use the lowest
value of "i" that has not already been used (successfully or not) to
negotiate resumption with the same host and for the same pre-session
key "ss[0]".
A session secret may not be used to secure more than one TCP
connection. To prevent this, a host MUST NOT resume with a session
secret if it has ever enabled encryption in the past with the same
secret, in either role. In the event that two hosts simultaneously
send SYN segments to each other that propose resumption with the same
session secret but the two segments are not part of a simultaneous
open, both connections will have to revert to fresh key-exchange. To
avoid this limitation, implementations MAY choose to implement
session resumption such that a given pre-session key "ss[0]" is only
used for either passive or active opens at the same host, not both.
If two hosts have previously negotiated a tcpcrypt session, either
host may later initiate session resumption regardless of which host
was the active opener or played the "A" role in the previous session.
However, a given host must either encrypt with keys "k_ab[j]" for all
sessions derived from the same pre-session key "ss[0]", or with keys
"k_ba[j]". Thus, which keys a host uses to send segments is not
affected by the role it plays in the current connection: it depends
only on whether the host played the "A" or "B" role in the initial
session.
Implementations that cache session secrets MUST provide a means for
applications to control that caching. In particular, when an
application requests a new TCP connection, it must be able to specify
that during the connection no session secrets will be cached and all
resumption requests will be ignored in favor of fresh key exchange.
And for an established connection, an application must be able to
cause any cache state that was used in or resulted from establishing
the connection to be flushed. A companion document
[I-D.ietf-tcpinc-api] describes recommended interfaces for this
purpose.
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3.6. Data Encryption and Authentication
Following key exchange (or its omission via session resumption), all
further communication in a tcpcrypt-enabled connection is carried out
within delimited _encryption frames_ that are encrypted and
authenticated using the agreed keys.
This protection is provided via algorithms for Authenticated
Encryption with Associated Data (AEAD). The particular algorithms
that may be used are listed in Table 3, and additional algorithms may
be specified according to the policy in Section 7. One algorithm is
selected during the negotiation described in Section 3.3.
The format of an encryption frame is specified in Section 4.2. A
sending host breaks its stream of application data into a series of
chunks. Each chunk is placed in the "data" portion of a "plaintext"
value, which is then encrypted to yield a frame's "ciphertext" field.
Chunks must be small enough that the ciphertext (whose length depends
on the AEAD cipher used, and is generally slightly longer than the
plaintext) has length less than 2^16 bytes.
An "associated data" value (see Section 4.2.2) is constructed for the
frame. It contains the frame's "control" field and the length of the
ciphertext.
A "frame nonce" value (see Section 4.2.3) is also constructed for the
frame but not explicitly transmitted. It contains an "offset" field
whose integer value is the zero-indexed byte offset of the beginning
of the current encryption frame in the underlying TCP datastream.
(That is, the offset in the framing stream, not the plaintext
application stream.) Because it is strictly necessary for the
security of the AEAD algorithms specified in this document, an
implementation MUST NOT ever transmit distinct frames with the same
nonce value under the same encryption key. In particular, a
retransmitted TCP segment MUST contain the same payload bytes for the
same TCP sequence numbers, and a host MUST NOT transmit more than
2^64 bytes in the underlying TCP datastream (which would cause the
"offset" field to wrap) before re-keying.
With reference to the "AEAD Interface" described in Section 2 of
[RFC5116], tcpcrypt invokes the AEAD algorithm with the secret key
"K" set to "k_ab[j]" or "k_ba[j]" for some "j", according to the
host's role as described in Section 3.3. The plaintext value serves
as "P", the associated data as "A", and the frame nonce as "N". The
output of the encryption operation, "C", is transmitted in the
frame's "ciphertext" field.
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When a frame is received, tcpcrypt reconstructs the associated data
and frame nonce values (the former contains only data sent in the
clear, and the latter is implicit in the TCP stream), and provides
these and the ciphertext value to the the AEAD decryption operation.
The output of this operation is either a plaintext value "P" or the
special symbol FAIL. In the latter case, the implementation MUST
either drop the TCP segment(s) containing the frame or abort the
connection; but if it aborts, the implementation MUST raise an error
condition distinct from the end-of-file condition.
3.7. TCP Header Protection
The "ciphertext" field of the encryption frame contains protected
versions of certain TCP header values.
When the "URGp" bit is set, the "urgent" value indicates an offset
from the current frame's beginning offset; the sum of these offsets
gives the index of the last byte of urgent data in the application
datastream.
A sender MUST set the "FINp" bit on the last frame it sends in the
connection (unless it aborts the connection), and MUST NOT set "FINp"
on any other frame.
TCP sets the FIN flag when a sender has no more data, which with
tcpcrypt means setting FIN on the segment containing the last byte of
the last frame. However, a receiver MUST report the end-of-file
condition to the connection's local user when and only when it
receives a frame with the "FINp" bit set. If a host receives a
segment with the TCP FIN flag set but the received datastream
including this segment does not contain a frame with "FINp" set, the
host SHOULD abort the connection and raise an error condition
distinct from the end-of-file condition. But if there are
unacknowledged segments whose retransmission could potentially result
in a valid frame, the host MAY instead drop the segment with the TCP
FIN flag set.
3.8. Re-Keying
Re-keying allows hosts to wipe from memory keys that could decrypt
previously transmitted segments. It also allows the use of AEAD
ciphers that can securely encrypt only a bounded number of messages
under a given key.
As described above in Section 3.3, a master key "mk[j]" is used to
generate two encryption keys "k_ab[j]" and "k_ba[j]". We refer to
these as a _key-set_ with _generation number_ "j". Each host
maintains a _local generation number_ that determines which key-set
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it uses to encrypt outgoing frames, and a _remote generation number_
equal to the highest generation used in frames received from its
peer. Initially, these two generation numbers are set to zero.
A host MAY increment its local generation number beyond the remote
generation number it has recorded. We call this action _initiating
re-keying_.
When a host has incremented its local generation number and uses the
new key-set for the first time to encrypt an outgoing frame, it MUST
set "rekey = 1" for that frame. It MUST set this field to zero in
all other cases.
When a host receives a frame with "rekey = 1", it increments its
record of the remote generation number. If the remote generation
number is now greater than the local generation number, the receiver
MUST immediately increment its local generation number to match.
Moreover, if the receiver has not yet transmitted a segment with the
FIN flag set, it MUST immediately send a frame (with empty
application data if necessary) with "rekey = 1".
A host SHOULD NOT initiate more than one concurrent re-key operation
if it has no data to send; that is, it should not initiate re-keying
with an empty encryption frame more than once while its record of the
remote generation number is less than its own.
Note that when parts of the datastream are retransmitted, TCP
requires that implementations always send the same data bytes for the
same TCP sequence numbers. Thus, frame data in retransmitted
segments must be encrypted with the same key as when it was first
transmitted, regardless of the current local generation number.
Implementations SHOULD delete older-generation keys from memory once
they have received all frames they will need to decrypt with the old
keys and have encrypted all outgoing frames under the old keys.
3.9. Keep-Alive
Instead of using TCP Keep-Alives to verify that the remote endpoint
is still responsive, tcpcrypt implementations SHOULD employ the re-
keying mechanism for this purpose, as follows. When necessary, a
host SHOULD probe the liveness of its peer by initiating re-keying
and transmitting a new frame immediately (with empty application data
if necessary).
As described in Section 3.8, a host receiving a frame encrypted under
a generation number greater than its own MUST increment its own
generation number and (if it has not already transmitted a segment
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with FIN set) immediately transmit a new frame (with zero-length
application data if necessary).
Implementations MAY use TCP Keep-Alives for purposes that do not
require endpoint authentication, as discussed in Section 8.2.
4. Encodings
This section provides byte-level encodings for values transmitted or
computed by the protocol.
4.1. Key-Exchange Messages
The "Init1" message has the following encoding:
byte 0 1 2 3
+-------+-------+-------+-------+
| INIT1_MAGIC |
| |
+-------+-------+-------+-------+
4 5 6 7
+-------+-------+-------+-------+
| message_len |
| = M |
+-------+-------+-------+-------+
8 K + 8
+--------+---------+---------+---...---+-----------+
|nciphers|sym- |sym- | |sym- |
| = K |cipher[0]|cipher[1]| |cipher[K-1]|
+--------+---------+---------+---...---+-----------+
K + 9 K + 9 + N_A_LEN
| |
v v
+-------+---...---+-------+-------+---...---+-------+
| N_A | PK_A |
| | |
+-------+---...---+-------+-------+---...---+-------+
M - 1
+-------+---...---+-------+
| ignored |
| |
+-------+---...---+-------+
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The constant "INIT1_MAGIC" is defined in Table 1. The four-byte
field "message_len" gives the length of the entire "Init1" message,
encoded as a big-endian integer. The "nciphers" field contains an
integer value that specifies the number of one-byte symmetric-cipher
identifiers that follow. The "sym-cipher" bytes identify
cryptographic algorithms in Table 3. The length "N_A_LEN" and the
length of "PK_A" are both determined by the negotiated TEP, as
described in Section 5.
Implementations of this protocol MUST construct "Init1" such that the
field "ignored" has zero length; that is, they must construct the
message such that its end, as determined by "message_len", coincides
with the end of the field "PK_A". When receiving "Init1", however,
implementations MUST permit and ignore any bytes following "PK_A".
The "Init2" message has the following encoding:
byte 0 1 2 3
+-------+-------+-------+-------+
| INIT2_MAGIC |
| |
+-------+-------+-------+-------+
4 5 6 7 8
+-------+-------+-------+-------+-------+
| message_len |sym- |
| = M |cipher |
+-------+-------+-------+-------+-------+
9 9 + N_B_LEN
| |
v v
+-------+---...---+-------+-------+---...---+-------+
| N_B | PK_B |
| | |
+-------+---...---+-------+-------+---...---+-------+
M - 1
+-------+---...---+-------+
| ignored |
| |
+-------+---...---+-------+
The constant "INIT2_MAGIC" is defined in Table 1. The four-byte
field "message_len" gives the length of the entire "Init2" message,
encoded as a big-endian integer. The "sym-cipher" value is a
selection from the symmetric-cipher identifiers in the previously-
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received "Init1" message. The length "N_B_LEN" and the length of
"PK_B" are both determined by the negotiated TEP, as described in
Section 5.
Implementations of this protocol MUST construct "Init2" such that the
field "ignored" has zero length; that is, they must construct the
message such that its end, as determined by "message_len", coincides
with the end of the "PK_B" field. When receiving "Init2", however,
implementations MUST permit and ignore any bytes following "PK_B".
4.2. Encryption Frames
An _encryption frame_ comprises a control byte and a length-prefixed
ciphertext value:
byte 0 1 2 3 clen+2
+-------+-------+-------+-------+---...---+-------+
|control| clen | ciphertext |
+-------+-------+-------+-------+---...---+-------+
The field "clen" is an integer in big-endian format and gives the
length of the "ciphertext" field.
The byte "control" has this structure:
bit 7 1 0
+-------+---...---+-------+-------+
| cres | rekey |
+-------+---...---+-------+-------+
The seven-bit field "cres" is reserved; implementations MUST set
these bits to zero when sending, and MUST ignore them when receiving.
The use of the "rekey" field is described in Section 3.8.
4.2.1. Plaintext
The "ciphertext" field is the result of applying the negotiated
authenticated-encryption algorithm to a "plaintext" value, which has
one of these two formats:
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byte 0 1 plen-1
+-------+-------+---...---+-------+
| flags | data |
+-------+-------+---...---+-------+
byte 0 1 2 3 plen-1
+-------+-------+-------+-------+---...---+-------+
| flags | urgent | data |
+-------+-------+-------+-------+---...---+-------+
(Note that "clen" in the previous section will generally be greater
than "plen", as the ciphertext produced by the authenticated-
encryption scheme must both encrypt the application data and provide
a way to verify its integrity.)
The "flags" byte has this structure:
bit 7 6 5 4 3 2 1 0
+----+----+----+----+----+----+----+----+
| fres |URGp|FINp|
+----+----+----+----+----+----+----+----+
The six-bit value "fres" is reserved; implementations MUST set these
six bits to zero when sending, and MUST ignore them when receiving.
When the "URGp" bit is set, it indicates that the "urgent" field is
present, and thus that the plaintext value has the second structure
variant above; otherwise the first variant is used.
The meaning of "urgent" and of the flag bits is described in
Section 3.7.
4.2.2. Associated Data
An encryption frame's "associated data" (which is supplied to the
AEAD algorithm when decrypting the ciphertext and verifying the
frame's integrity) has this format:
byte 0 1 2
+-------+-------+-------+
|control| clen |
+-------+-------+-------+
It contains the same values as the frame's "control" and "clen"
fields.
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4.2.3. Frame Nonce
Lastly, a "frame nonce" (provided as input to the AEAD algorithm) has
this format:
byte
+------+------+------+------+
0 | FRAME_NONCE_MAGIC |
+------+------+------+------+
4 | |
+ offset +
8 | |
+------+------+------+------+
The 4-byte magic constant is defined in Table 1. The 8-byte "offset"
field contains an integer in big-endian format. Its value is
specified in Section 3.6.
4.3. Constant Values
The table below defines values for the constants used in the
protocol.
+------------+-------------------+
| Value | Name |
+------------+-------------------+
| 0x01 | CONST_NEXTK |
| 0x02 | CONST_SESSID |
| 0x03 | CONST_REKEY |
| 0x04 | CONST_KEY_A |
| 0x05 | CONST_KEY_B |
| 0x06 | CONST_RESUME |
| 0x15101a0e | INIT1_MAGIC |
| 0x097105e0 | INIT2_MAGIC |
| 0x44415441 | FRAME_NONCE_MAGIC |
+------------+-------------------+
Table 1: Constant values used in the protocol
5. Key-Agreement Schemes
The TEP negotiated via TCP-ENO indicates the use of one of the key-
agreement schemes named in Table 2. For example,
"TCPCRYPT_ECDHE_P256" names the tcpcrypt protocol using ECDHE-P256
together with the CPRF and length parameters specified below.
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All the TEPs specified in this document require the use of HKDF-
Expand-SHA256 as the CPRF, and these lengths for nonces and session
keys:
N_A_LEN: 32 bytes
N_B_LEN: 32 bytes
K_LEN: 32 bytes
If future documents assign additional TEPs for use with tcpcrypt,
they may specify different values for the lengths above. Note that
the minimum session ID length required by TCP-ENO, together with the
way tcpcrypt constructs session IDs, implies that "K_LEN" must have
length at least 32 bytes.
Key-agreement schemes ECDHE-P256 and ECDHE-P521 employ the ECSVDP-DH
secret value derivation primitive defined in [ieee1363]. The named
curves are defined in [nist-dss]. When the public-key values "PK_A"
and "PK_B" are transmitted as described in Section 4.1, they are
encoded with the "Elliptic Curve Point to Octet String Conversion
Primitive" described in Section E.2.3 of [ieee1363], and are prefixed
by a two-byte length in big-endian format:
byte 0 1 2 L - 1
+-------+-------+-------+---...---+-------+
| pubkey_len | pubkey |
| = L | |
+-------+-------+-------+---...---+-------+
Implementations SHOULD encode these "pubkey" values in "compressed
format", and MUST accept values encoded in "compressed",
"uncompressed" or "hybrid" formats.
Key-agreement schemes ECDHE-Curve25519 and ECDHE-Curve448 use the
functions X25519 and X448, respectively, to perform the Diffie-Helman
protocol as described in [RFC7748]. When using these ciphers,
public-key values "PK_A" and "PK_B" are transmitted directly with no
length prefix: 32 bytes for Curve25519, and 56 bytes for Curve448.
The TEP "TCPCRYPT_ECDHE_Curve25519" is mandatory to implement. This
means a tcpcrypt implementation MUST support at least this TEP,
although system administrators need not enable it.
6. AEAD Algorithms
Specifiers and key-lengths for AEAD algorithms are given in Table 3.
The algorithms "AEAD_AES_128_GCM" and "AEAD_AES_256_GCM" are
specified in [RFC5116]. The algorithm "AEAD_CHACHA20_POLY1305" is
specified in [RFC7539].
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7. IANA Considerations
Tcpcrypt's TEP identifiers will need to be incorporated in IANA's
"TCP encryption protocol identifiers" registry under the
"Transmission Control Protocol (TCP) Parameters" registry, as in the
following table. The various key-agreement schemes used by these
tcpcrypt variants are defined in Section 5.
+-------+---------------------------+-----------+
| Value | Meaning | Reference |
+-------+---------------------------+-----------+
| 0x21 | TCPCRYPT_ECDHE_P256 | [RFC-TBD] |
| 0x22 | TCPCRYPT_ECDHE_P521 | [RFC-TBD] |
| 0x23 | TCPCRYPT_ECDHE_Curve25519 | [RFC-TBD] |
| 0x24 | TCPCRYPT_ECDHE_Curve448 | [RFC-TBD] |
+-------+---------------------------+-----------+
Table 2: TEP identifiers for use with tcpcrypt
In Section 4.1, this document defines "sym-cipher" specifiers for
which IANA is to maintain a new "tcpcrypt AEAD Algorithm" registry
under the "Transmission Control Protocol (TCP) Parameters" registry,
with initial values as given in the following table. The AEAD
algorithms named there are defined in Section 6. Future assignments
are to be made under the "RFC Required" policy detailed in [RFC8126],
relying on early allocation [RFC7120] to facilitate testing before an
RFC is finalized.
+-------+------------------------+------------+-----------+
| Value | AEAD Algorithm | Key Length | Reference |
+-------+------------------------+------------+-----------+
| 0x01 | AEAD_AES_128_GCM | 16 bytes | [RFC-TBD] |
| 0x02 | AEAD_AES_256_GCM | 32 bytes | [RFC-TBD] |
| 0x10 | AEAD_CHACHA20_POLY1305 | 32 bytes | [RFC-TBD] |
+-------+------------------------+------------+-----------+
Table 3: Authenticated-encryption algorithms corresponding to sym-
cipher specifiers in Init1 and Init2 messages.
8. Security Considerations
Public-key generation, public-key encryption, and shared-secret
generation all require randomness. Other tcpcrypt functions may also
require randomness, depending on the algorithms and modes of
operation selected. A weak pseudo-random generator at either host
will compromise tcpcrypt's security. Many of tcpcrypt's
cryptographic functions require random input, and thus any host
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implementing tcpcrypt MUST have access to a cryptographically-secure
source of randomness or pseudo-randomness.
Most implementations will rely on a device's pseudo-random generator,
seeded from hardware events and a seed carried over from the previous
boot. Once a pseudo-random generator has been properly seeded, it
can generate effectively arbitrary amounts of pseudo-random data.
However, until a pseudo-random generator has been seeded with
sufficient entropy, not only will tcpcrypt be insecure, it will
reveal information that further weakens the security of the pseudo-
random generator, potentially harming other applications. As
required by TCP-ENO, implementations MUST NOT send ENO options unless
they have access to an adequate source of randomness.
The cipher-suites specified in this document all use HMAC-SHA256 to
implement the collision-resistant pseudo-random function denoted by
"CPRF". A collision-resistant function is one for which, for
sufficiently large L, an attacker cannot find two distinct inputs
"K_1", "CONST_1" and "K_2", "CONST_2" such that "CPRF(K_1, CONST_1,
L) = CPRF(K_2, CONST_2, L)". Collision resistance is important to
assure the uniqueness of session IDs, which are generated using the
CPRF.
All of the security considerations of TCP-ENO apply to tcpcrypt. In
particular, tcpcrypt does not protect against active eavesdroppers
unless applications authenticate the session ID. If it can be
established that the session IDs computed at each end of the
connection match, then tcpcrypt guarantees that no man-in-the-middle
attacks occurred unless the attacker has broken the underlying
cryptographic primitives (e.g., ECDH). A proof of this property for
an earlier version of the protocol has been published [tcpcrypt].
To gain middlebox compatibility, tcpcrypt does not protect TCP
headers. Hence, the protocol is vulnerable to denial-of-service from
off-path attackers just as plain TCP is. Possible attacks include
desynchronizing the underlying TCP stream, injecting RST or FIN
segments, and forging rekey bits. These attacks will cause a
tcpcrypt connection to hang or fail with an error, but not in any
circumstance where plain TCP could continue uncorrupted.
Implementations MUST give higher-level software a way to distinguish
such errors from a clean end-of-stream (indicated by an authenticated
"FINp" bit) so that applications can avoid semantic truncation
attacks.
There is no "key confirmation" step in tcpcrypt. This is not
required because tcpcrypt's threat model includes the possibility of
a connection to an adversary. If key negotiation is compromised and
yields two different keys, all subsequent frames will be ignored due
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to failed integrity checks, causing the application's connection to
hang. This is not a new threat because in plain TCP, an active
attacker could have modified sequence and acknowledgement numbers to
hang the connection anyway.
Tcpcrypt uses short-lived public keys to provide forward secrecy.
That is, once an implementation removes these keys from memory, a
compromise of the system will not provide any means to derive the
session keys for past connections. All currently-specified key
agreement schemes involve ECDHE-based key agreement, meaning a new
keypair can be efficiently computed for each connection. If
implementations reuse these parameters, they SHOULD limit the
lifetime of the private parameters as far as practical in order to
minimize the number of past connections that are vulnerable.
Attackers cannot force passive openers to move forward in their
session resumption chain without guessing the content of the
resumption identifier, which will be difficult without key knowledge.
8.1. Asymmetric Roles
Tcpcrypt transforms a shared pseudo-random key (PRK) into
cryptographic session keys for each direction. Doing so requires an
asymmetry in the protocol, as the key derivation function must be
perturbed differently to generate different keys in each direction.
Tcpcrypt includes other asymmetries in the roles of the two hosts,
such as the process of negotiating algorithms (e.g., proposing vs.
selecting cipher suites).
8.2. Verified Liveness
Many hosts implement TCP Keep-Alives [RFC1122] as an option for
applications to ensure that the other end of a TCP connection still
exists even when there is no data to be sent. A TCP Keep-Alive
segment carries a sequence number one prior to the beginning of the
send window, and may carry one byte of "garbage" data. Such a
segment causes the remote side to send an acknowledgment.
Unfortunately, tcpcrypt cannot cryptographically verify Keep-Alive
acknowledgments. Hence, an attacker could prolong the existence of a
session at one host after the other end of the connection no longer
exists. (Such an attack might prevent a process with sensitive data
from exiting, giving an attacker more time to compromise a host and
extract the sensitive data.)
To counter this threat, tcpcrypt specifies a way to stimulate the
remote host to send verifiably fresh and authentic data, described in
Section 3.9.
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The TCP keep-alive mechanism has also been used for its effects on
intermediate nodes in the network, such as preventing flow state from
expiring at NAT boxes or firewalls. As these purposes do not require
the authentication of endpoints, implementations may safely
accomplish them using either the existing TCP keep-alive mechanism or
tcpcrypt's verified keep-alive mechanism.
8.3. Mandatory Key-Agreement Schemes
This document mandates that tcpcrypt implementations provide support
for at least one key-agreement scheme: ECDHE using Curve25519. This
choice of a single mandatory algorithm is the result of a difficult
tradeoff between cryptographic diversity and the ease and security of
actual deployment.
The IETF's appraisal of best current practice on this matter
[RFC7696] says, "Ideally, two independent sets of mandatory-to-
implement algorithms will be specified, allowing for a primary suite
and a secondary suite. This approach ensures that the secondary
suite is widely deployed if a flaw is found in the primary one."
To meet that ideal, it might appear natural to also mandate ECDHE
using P-256, as this scheme is well-studied, widely implemented, and
sufficiently different from the Curve25519-based scheme that it is
unlikely they will both suffer from a single cryptoanalytic advance.
However, implementing the Diffie-Hellman function using NIST elliptic
curves (including those specified for use with tcpcrypt, P-256 and
P-521) appears to be very difficult to achieve without introducing
vulnerability to side-channel attacks [nist-ecc]. Although well-
trusted implementations are available as part of large cryptographic
libraries, these may be difficult to extract for use in operating-
system kernels where tcpcrypt is usually best implemented. In
contrast, the characteristics of Curve25519 together with its recent
popularity has led to many safe and efficient implementations,
including some that fit naturally into the kernel environment.
[RFC7696] insists that, "The selected algorithms need to be resistant
to side-channel attacks and also meet the performance, power, and
code size requirements on a wide variety of platforms." On this
principle, tcpcrypt excludes the NIST curves from the set of
mandatory-to-implement key-agreement algorithms.
9. Acknowledgments
We are grateful for contributions, help, discussions, and feedback
from the TCPINC working group, including Marcelo Bagnulo, David
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Black, Bob Briscoe, Jana Iyengar, Tero Kivinen, Mirja Kuhlewind, Yoav
Nir, Christoph Paasch, Eric Rescorla, and Kyle Rose.
This work was funded by gifts from Intel (to Brad Karp) and from
Google; by NSF award CNS-0716806 (A Clean-Slate Infrastructure for
Information Flow Control); by DARPA CRASH under contract
#N66001-10-2-4088; and by the Stanford Secure Internet of Things
Project.
10. Contributors
Dan Boneh and Michael Hamburg were co-authors of the draft that
became this document.
11. References
11.1. Normative References
[I-D.ietf-tcpinc-tcpeno]
Bittau, A., Giffin, D., Handley, M., Mazieres, D., and E.
Smith, "TCP-ENO: Encryption Negotiation Option", draft-
ietf-tcpinc-tcpeno-11 (work in progress), October 2017.
[ieee1363]
IEEE, "IEEE Standard Specifications for Public-Key
Cryptography (IEEE Std 1363-2000)", 2000.
[nist-dss]
NIST, "FIPS PUB 186-4: Digital Signature Standard (DSS)",
2013.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997, .
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, .
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
.
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[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010, .
[RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code
Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
2014, .
[RFC7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,
.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, .
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, .
11.2. Informative References
[I-D.ietf-tcpinc-api]
Bittau, A., Boneh, D., Giffin, D., Handley, M., Mazieres,
D., and E. Smith, "Interface Extensions for TCP-ENO and
tcpcrypt", draft-ietf-tcpinc-api-05 (work in progress),
September 2017.
[nist-ecc]
Bernstein, D. and T. Lange, "Failures in NIST's ECC
standards", 2016, .
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989, .
[RFC7696] Housley, R., "Guidelines for Cryptographic Algorithm
Agility and Selecting Mandatory-to-Implement Algorithms",
BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
.
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[tcpcrypt]
Bittau, A., Hamburg, M., Handley, M., Mazieres, D., and D.
Boneh, "The case for ubiquitous transport-level
encryption", USENIX Security , 2010.
Authors' Addresses
Andrea Bittau
Google
345 Spear Street
San Francisco, CA 94105
US
Email: bittau@google.com
Daniel B. Giffin
Stanford University
353 Serra Mall, Room 288
Stanford, CA 94305
US
Email: dbg@scs.stanford.edu
Mark Handley
University College London
Gower St.
London WC1E 6BT
UK
Email: M.Handley@cs.ucl.ac.uk
David Mazieres
Stanford University
353 Serra Mall, Room 290
Stanford, CA 94305
US
Email: dm@uun.org
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Quinn Slack
Sourcegraph
121 2nd St Ste 200
San Francisco, CA 94105
US
Email: sqs@sourcegraph.com
Eric W. Smith
Kestrel Institute
3260 Hillview Avenue
Palo Alto, CA 94304
US
Email: eric.smith@kestrel.edu
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